1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuit fabrication, and more specifically to a flip-chip integrated circuit comprising a plurality of interconnect bumps, each including an integral standoff and an hourglass shaped solder coating.
2. Description of the Related Art
The "flip-chip" type of integrated circuit interconnection enables a substantial increase in the number and density of interconnections that can be made between an integrated circuit chip and a supporting printed circuit board (PCB) by eliminating wire bonds therebetween. The flip-chip configuration is also applicable to interconnectably mounting one chip on another chip.
A flip-chip mounting and interconnect system that is known as C4 controlled collapse connection is described, for example, in U.S. Pat. No. 4,545,610, entitled "METHOD FOR FORMING ELONGATED SOLDER CONNECTIONS BETWEEN A SEMICONDUCTOR DEVICE AND A SUPPORTING SUBSTRATE", issued Oct. 8, 1985 to Mark Lakritz et al.
In this system, an electrically conductive "bump" is formed on each external connection or bond pad of an interconnect metallization pattern on the chip. A conjugate set of bumps is formed on the PCB. The chip is placed face down on the PCB with the bumps pressed together. Solder that is coated on either or both sets of bumps is heated and caused to reflow, and provide ohmic connection between the sets of bumps.
In the basic C4 system, however, since the bumps are formed of solder, they tend to collapse under applied pressure during reflow, creating a pancake shape that tends to concentrate thermal stresses on the solder bonds. The various materials of which the integrated circuit chip is fabricated have different coefficients of thermal expansion. During thermal cycling such as typically experienced by the circuit during operation, the different materials expand and contract at different rates. This mismatch creates stresses that produce fatigue within the solder bonds and can result in catastrophic failure of the interconnections.
For this reason, rigid spacers or standoffs are conventionally provided between the chip and the PCB or other structure on which the chip is mounted. The standoffs are typically metal spheres having a diameter approximately equal to the desired spacing between the chip and the support.
The standoffs relieve a large part of the thermal stresses applied to the solder bonds. However, their provision requires additional complicated process steps, and care must be taken to ensure that the standoffs do not become loose and short out portions of the circuit.
An improvement to the basic flip-chip bump interconnect system is disclosed in U.S. Pat. No. 5,154,341, entitled "NONCOLLAPSING MULTISOLDER INTERCONNECTION", issued Oct. 13, 1992 to Cynthia Melton et al. In this system, relatively rigid bumps of a high temperature solder are formed on the interconnect metallization pattern of a flip-chip integrated circuit, and balls of soft, low temperature solder are formed on conjugate bonding pads of the PCB. The chip is positioned and pressed against the PCB with the bumps spaced closely adjacent to the solder balls.
Heat is applied to reflow the solder, such that the low temperature solder from the ball coats the high temperature solder bump. Due to the geometry of the configuration, the solder coating assumes an hourglass shape that, in combination with the relatively rigid high temperature solder bump that functions as a standoff, relieves thermal stresses on the solder bond.
Although providing a desirable interconnect configuration, the method disclosed by Melton is disadvantageous in that the bump and solder ball must be formed separately, and precise alignment must be provided such that the bump and the ball are laterally spaced from each other by the correct distance.